<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>新增伪类</title>
    <style>
        input:hover {
            color: red;
        }

        input:focus:nth-of-type(1) {
            background-color: orange;
        }

        input:focus:nth-of-type(2) {
            background-color: yellow;
        }

        div {
            width: 200px;
            height: 150px;
            background-color: #eee;
            margin-top: 5px;
        }

        section {
            width: 200px;
            height: 30px;
            background-color: #ff0;
            margin-bottom: 5px;
        }

        div:empty {
           background-color: red;
        }

        section:empty{
           background-color:  blue;
        }
    </style>
</head>

<body>
    <div>
        <section>
            用户名: <input type="text"></section>
        <section> 密码：<input type="password"></section>
    </div>
    <div></div>
    <section>three</section>
    <section>four</section>
    <section>five</section>
    <section></section>
</body>

</html>